1. Field of the Invention
The invention relates to a hydraulic circuit, comprising at least two double acting piston cylinder devices, and more particularly to such a circuit for opening and closing a folding roof of a vehicle of the cabriolet type.
2. Discussion of the Prior Art
Hydraulic circuits of the aforementioned type typically have a reservoir for hydraulic fluid, an electric motor driven hydraulic pump connected therewith and, for each of the piston cylinder devices, an electrically operated valve that is connected to the space under the piston of the respective piston cylinder device so as to selectively connect the space either to the pump or to the reservoir.
In the above description the term "cylinder space under the piston" should be interpreted as the cylinder space at the end turned away from the piston rod, whereas "cylinder space above the piston" should be interpreted as the cylinder space at the piston rod end of the cylinder.
Such a circuit is well-known for hydraulically opening and closing folding roofs with vehicles of the cabriolet type.
As an example, reference may be made to U.S. Pat. No. 5,454,292, issued Oct. 3, 1995. Three (pairs of) piston cylinder devices are used therein, which have to be activated in a certain sequence. In such a case, one speaks of a hydraulic circuit with a number of cylinder functions (three in the case referred to).
With this well-known circuit, the activation is effected by an irreversible pump in cooperation with a control valve block that comprises two valves for each (double acting) cylinder function. Such valves are relatively expensive and render a hydraulic device for opening and closing of a folding roof with a vehicle of the cabriolet type rather expensive, dependent on the number of cylinder functions to be activated.
The invention aims at meeting this drawback of the well-known circuit.